The present application claims priority from Japanese Patent Applications No. 2014-206750 and No. 2014-206749, filed Oct. 7, 2014, which are incorporated herein by reference.
The present invention generally relates to a two-stroke internal-combustion engine and more specifically relates to an air leading-type engine that first induces air to flow into a combustion chamber in a scavenging stroke.
Two-stroke internal-combustion engines of the type in which scavenging is performed using air-fuel mixture are often used in portable work machines such as brush cutters and chain saws. This type of two-stroke internal-combustion engine includes a scavenging channel that brings a crankcase and a combustion chamber into communication with each other. Air-fuel mixture pre-compressed in the crankcase is induced to flow into the combustion chamber through the scavenging channel, and scavenging is performed by the air-fuel mixture.
As well-known, two-stroke engines have the problem of “air-fuel mixture (new gas) blow-by”. In response to this problem, air leading-type stratified scavenging two-stroke internal-combustion engines have been proposed and already put into practical use (U.S. Pat. No. 6,857,402). In an air leading-type stratified scavenging engine, air is charged into a scavenging channel in advance. In a scavenging stroke, first, the air accumulated in the scavenging channel is induced to flow into a combustion chamber and then air-fuel mixture in a crankcase is induced to flow into the combustion chamber through the scavenging channel.
FIG. 8 is a diagram relating to opening/closing of a port in a conventional air leading-type stratified scavenging engine. In FIG. 8, in order to avoid confusion of drawn lines, illustration of a piston is omitted. In the figure, reference numeral 100 denotes a cylinder wall. In the cylinder wall 100, an air channel 102 and an air-fuel mixture channel (not shown) open. Air-fuel mixture is supplied to a crankcase through the air-fuel mixture channel. An air port of the air channel 102 is denoted by reference numeral 102a. Also, in the cylinder wall 100, a scavenging port 104a of a scavenging channel 104 opens. The scavenging channel 104 communicates with a crankcase. Each of the air port 102a and the scavenging port 104a is opened/closed by the piston. The piston has a groove 106 in a peripheral surface thereof. The piston groove 106 extends in a circumferential direction.
(I) to (III) of FIG. 8 chronologically illustrate states in the course of the piston moving up toward the top dead center. (II) of FIG. 8 indicates a state in which the piston moves up relative to the position in (I) of FIG. 8. (III) of FIG. 8 indicates a state in which the piston moves up relative to the position in (II) of FIG. 8.
Referring to (I) of FIG. 8, immediately before the piston groove 106 reaches the air port 102a after the piston moving up from the bottom dead center toward the top dead center, a gas blown back in previous scavenging is mixed in the piston groove 106. The blown-back gas contains air-fuel mixture components. The blown-back gas remaining in the piston groove 106 is indicated by dots. In (II) of FIG. 8, which illustrates a state in which the piston further moves up toward the top dead center, the piston groove 106 communicates with the air port 102a. In the state in (II) of FIG. 8, the piston groove 106 is not in communication with the scavenging port 104a. Therefore, even though the piston groove 106 communicates with the air port 102a, no air flow from the air port 102a to the piston groove 106 is generated at this point.
In (III) of FIG. 8, which illustrates a state in which the piston further moves up toward the top dead center, the piston groove 106 communicates with the air port 102a and also communicates with the scavenging port 104a. In this state in (III) of FIG. 8, air is charged into the scavenging channel 104.
In theory, in a conventional air leading-type stratified scavenging two-stroke internal-combustion engine, a flow of gas in the piston groove 106 occurs only when the piston groove 106 communicates with the scavenging port 104a. Then, the gas in the piston groove 106 first enters the scavenging channel 104, and then air enters from the air port 102a to the scavenging channel 104 through the piston groove 106 ((III) of FIG. 8). Therefore, a timing of the air entering the scavenging channel 104 from the piston groove 106 is later than a timing of the piston groove 106 starting communicating with the scavenging channel 104.
As well-known, a two-stroke internal-combustion engine for a work machine is run at a high rotation rate of, for example, 10,000 rpm. Therefore, the aforementioned timing delay largely affects the efficiency of air charge into a scavenging channel 104. More specifically, two-stroke internal-combustion engines for work machines have the essential problem of difficulty in ensuring the certainty of charging air into the scavenging channel 104 in each cycle. In order to address this problem, in reality, conventional air leading-type stratified scavenging two-stroke internal-combustion engines employ a configuration in which a timing for a piston groove 106 to come into communication with a scavenging port 104a is substantially advanced. However, employment of this configuration results in air-fuel mixture components remaining in a gas scavenging channel 104 easily flowing to the air channel 102 side, which causes decrease in emission characteristic improvement effect.
An object of the present invention is to provide an air leading-type stratified scavenging two-stroke internal-combustion engine that can enhance the efficiency of charging air to a scavenging channel by generating a gas flow in a piston groove simultaneously with the piston groove coming into communication with an air port.